The invention relates generally to a disconnect switch and more particularly to a disconnect switch for metal-clad, pressurized gas insulated, high voltage switchgear installations having 2 cylindrical contacts which may be surrounded by field electrodes and which connect to each other in the closed position, for which purpose at least one of the contacts is movable along a common longitudinal axis.
A disconnect switch is disclosed in GB Pat. No. 15 44 398 in which inside of one contact there is arranged a movable isolating tube, which, during the switching operation bridges the isolation space, when the contacts are moved. The isolating tube bridges the isolation space in an arc-impervious way, before one contact enters into galvanic contact with the opposite contact. Movement of the isolating tube ist effected by the mowement of the contact. This will prevent any preliminary arc discharge, between the contacts when still at a certain distance from one another, which might develop when switching the disconnect switch under voltage, from wandering during slow switch movements and from sparking over to the grounded metal housing. The isolating tube thus becomes a pre-arcing cage, bridging the isolation space, before arcing distance by the contacts is reached. At the return of the contact, the isolating tube leaves the isolation space only after the moving contact is within reach of the shielding electrode and no arcing is possible.
It is known for a disconnect switch, from French Pat. No. 1,514,265, to arrange a cylindrical resistance on the inside of the isolating housing surrounding the switching chamber. This resistance at one end is constantly in touch with one contact of the disconnect switch. The other end is connected to a contact rail running over a certain length on the inside of the insulating housing. The movable contact has an auxiliary contact that can slide on the contact rail. In the closed position of the disconnect switch, the auxiliary contact is lying on the contact rail in such a way that the resistance is in parallel to the contacts. This condition exists during the start of the opening movement of the movable contact until the auxiliary contact is leaving the contact rail. Based on the dimensions of the switch, this occurs only when the switching arc ist already extinguished. Then the resistance is no longer in parallel to the opened contacts. During the closing of this disconnect switch, the resistance again will be connected in parallel before the contacts touch.
It is known from U.S. Pat. No. 3,829,707 that high frequency oscillation can be generated during switching operations. Broad-band high frequency oscillations are generated in particular during switching of a disconnect switch with slow moving contacts within a pressurized-gas insulated, metal-clad, high-voltage switchgear installation. On the known metal-clad high-voltage circuit insulated with SF.sub.6, these oscillations are considerably suppressed or damped because the conducting element is covered along at least a part of its length with a high frequency dampening layer. This layer opposes the high frequency oscillations without influencing the currents flowing underneath in the conductor material with a normal operating frequency.
The invention also concernes the problem of high frequency oscillation in pressurized-gas insulated, metal-clad, high-voltage switchgear installation. It was recognized that under certain conditions, the frequencies of some of these broad-band high frequency oscillations could be in resonance with the characteristic frequencies of a metal-clad high-voltage switchgear installation resulting from its own dimensions. In such a case, stationary waves are generated inside the metal-clad high-voltage switchgear installation, the local current maxima of which reduce the insulating strength so much that an arcing to the metal of the encapsulation can take place.
It is an object of this invention to provide a disconnect switch for a metal-clad, pressurized-gas insulated, high-voltage switchgear installation wherein arcing to the metal of the encapsulation does not occur.
It is also an object of this invention to provide a disconnect switch which is not susceptible to high frequency resonance oscillations which reduce insulating strength encouraging arcing to the metal of the encapsulation.